Before discussing the four forces further, it will be helpful
to define some of the terms used extensively in this section.

• Acceleration—the force involved in overcoming inertia, and which is
defined as a change of velocity per unit of time.

• Airfoil—any surface designed to obtain reaction such as lift from
the air through which it moves.

• Angle of Attack—the angle between the chord line of the wing and the
direction of the relative wind. [Figure 1-2]

• Angle of Incidence—the angle formed by the chord line of the wing
and the longitudinal axis of the airplane. It is determined
during the design of the airplane and is the angle at which
the wing is attached to the fuselage. Therefore, it is a fixed angle and
cannot be changed by the pilot. Angle of incidence should not
be confused with angle of attack. [Figure 1-3]

• Camber—the curvature of the airfoil from the leading edge to the trailing
edge. “Upper camber” refers to the curvature of the upper surface; “lower
camber” refers to the curvature of the lower surface; and “mean camber”
refers to the mean line which is equidistant at all points between the
upper and lower surfaces. [Figure 1-4]

• Chord—an imaginary straight line drawn from the leading edge to the
trailing edge of a cross section of an airfoil. [Figure
1-3]

• Component—one of the various forces or parts of a combination of forces.
Figure 1-5 illustrates the component of lift
vertically and the component of drag horizontally.

• Relative Wind—the direction of the airflow produced by an object moving
through the air. The relative wind for an airplane in flight flows in a
direction parallel with and opposite to the direction of flight. Therefore,
the actual flightpath of the airplane determines the direction of the relative
wind. [Figure 1-6]

• Speed—the distance traveled in a given time.

• Vectors—the graphic representation of a force drawn in a straight
line which indicates direction by an arrow and magnitude by its length.
When an object is being acted upon by two or more forces, the combined
effect of these forces may be represented by a resultant vector. After
the vectors have been resolved, the resultant may be measured to determine
the direction and magnitude of the combined forces. [Figure
1-7]

• Velocity—the speed or rate of movement in a certain direction.

• Wing Area—the total surface of the wing (square feet), which includes
control surfaces and may include wing area covered by the fuselage (main
body of the airplane), and engine nacelles.

• Wing Planform—the shape or form of a wing as viewed from above. It
may be long and tapered, short and rectangular, or various other shapes.
[Figure 1-8]